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| Voltage AFE with fast rising edge problem |
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| Mark:
Hello, I am having a problem with an AFE (attached) that uses an INA826 instrumentation amplifier to drive an ADS1271 ADC. This is a single 5V supply circuit with a 2.5V reference. The entire AFE channel is powered by an isolating DC-DC and the digital signals also go across the isolation barrier. There is a voltage divider formed from two legs, each 100k and a divider resistor of 806 ohms to give a full scale output of +/-1.7V (referenced to the 2.5V mid point) for a 300VRMS input. The ADC uses a 1.7V reference. The measured voltage input can be 300VRMS (60Hz + harmonics) and can come from anything really. With sine waves, everything looks fine at the output, but with fast rising edges it falls apart, e.g. a differential square wave starts to show an anomaly at the zero crossing. I added some unbalanced stray capacitance between the inputs to the circuit ground and now the simulation shows the same waveforms as I'm seeing on the PCB (so I think I'm in the right area). The two x 475k resistors are intended to keep the common mode of the inputs at mid-supply. These should also provide a return path for the input bias currents for the INA826? Any ideas or suggestions welcome, thank you. |
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